Discuss Ohm value of transformer in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Transposition towers are semi common in the US, so I am familiar with that concept.

But going back- 6-500 kcmils in a 4 inch steal conduit. Or 32 600kcmil sets in 8 separate PVC conduits.

If anyone is curious, here are US cable sizes in mm2:

View: https://Upload the image directly to the thread.com/a/lZvLk2D


500kcmil = 253mm2

600kcmil= 304mm2
 
Transposition towers are semi common in the US, so I am familiar with that concept.

But going back- 6-500 kcmils in a 4 inch steal conduit. Or 32 600kcmil sets in 8 separate PVC conduits.
The sort of places I have seen conductors that large tend to have them on open trays, probably for ease of installation as much as for air cooling. But equally it is not really my area.
 
Last edited:
The sort of places I have seen conductors that large tend to have them on open trays, probably for ease of installation as much as for air cooling. But equally it is not really my area.


In the EU I've heard that being the norm, but in North America conduit- (either steal or PVC)- rains supreme.

Here is an example of a chiller breaker feeding two parallel runs in separate conduit:

View: https://Upload the image directly to the thread.com/JPeRNMq


US power companies deliver 120/208Y to large buildings as much as 277/480Y, so high current circuits (and their up-size for voltage drop) is very common. That and NEC article 220 load calcs tend to make feeders and services near double their actual peak current draw.
 
Last edited:
It looks like they keep the 3 phases in each conduit, presumably to reduce magnetic interaction a bit.

However the earth wires look astonishingly thin in comparison so I presume they are using a RCD for earth fault protection!?
 
It looks like they keep the 3 phases in each conduit, presumably to reduce magnetic interaction a bit.

Good eyes- correct. IF the conduit is steal it must be done that way.

The only typical exception is PVC conduit when installed between the padmount transformers and the service disconnecting means.

However the earth wires look astonishingly thin in comparison so I presume they are using a RCD for earth fault protection!?

No RCD. We struggle earth fault loop impedance lol. :flushed: :flushed:

Earth wires are sized based on NEC Table 250.122:

View: https://Upload the image directly to the thread.com/YQzWJKS


These values are basically calculated off the adiabatic method... the bare minimum size that won't cause the insulation to melt off during a ground fault.

So translating:

15 amp fuse or breaker requires 2.08mm2 earth wire

20 amp fuse or breaker requires 3.31mm2 earth wire

Here is where it gets rather small:

60 amp = 5.26mm2

100 amp = 8.36mm2

200 amp = 13.30mm2

300 amp = 21.15mm2

400 amp = 26.67mm2

500 amp = 33.62mm2

600 amp = 42.41mm2

800 amp = 53.49mm2

1000 amp = 67.43mm2
 
These values are basically calculated off the adiabatic method... the bare minimum size that won't cause the insulation to melt off during a ground fault.
But if (as you say) they don't really check Zs, how do they know the let-through I2t for the adiabatic calculation?
 
But if (as you say) they don't really check Zs, how do they know the let-through I2t for the adiabatic calculation?

Can you educate me on this concept? I have no idea how let-through in relations to Zs works in regards to EGCs.
[automerge]1590333865[/automerge]
If curious in regards to me starting this thread I ask based on 250.4 (A) (5) and 250.4 (B) (4) as indicated in Table 250.122:

View: https://Upload the image directly to the thread.com/t5RbR6p


Code mandates that impedance be low enough to trip the breaker- but does not specify the maximum time or method to determine that hence why I am turning to IEC 60364-4-41 Table 41.1 and BS7671 as a guide.

I know asking about IEC standards while simultaneously working with NFPA-70 is creating some confusion so I hope I can clear that up. Plus its fun learning new stuff :)
 
Last edited:
Basically if you have a given fault, and lets assume it is a dead short at the end of your circuit, then if you know Zs you can find the lowest PFC from the minimum supply voltage as:
PFC = Umin / Zs
From this level of fault current you then look up the current-time curve of the breaker or fuse and that gives you the time to disconnect at that level. Once you know the fault current and corresponding time you have the let-through I2t "energy" and that allows you to compute what it will do to the cable.

The usual assumption is the adiabatic case - then the input heat energy is over a short period of time so the out going heat energy can be neglected, which is reasonable for a short circuit type of fault - and so if you assume a closed thermal system, then a given temperature rise depends on the material constant and input energy.

This is normally simplified even more by assuming only a couple standard conductors (Cu, Al, and Fe) and a few common thermal limits (e.g. for thermoplastic and for thermosetting insulation) as a table of a few fudge-factors to use.

It might be easier to give an example. Say you have 63A fuse (common for domestic in UK) and you find your Ze value was 0.35 ohms (typical upper limit for TN-C-S) and you wanted to size a copper earth wire. Lowest fault current would be 95% of U / Ze

PFC = 0.95 * 230 / 0.35 = 624A

Here is a typical fuse curve:
Ohm value of transformer fuse-curve - EletriciansForums.net
If you look up 624A prospective current for the 63A curve (as far as practical) you see the corresponding time is around 0.4 seconds, so we can compute our let-through energy as:
I2t = 624^2 * 0.4 = 156k (A2s units)

Looking at the values for copper and say 30C initial and thermoplastic (70C cable) we have k = 143 so we can size our earth using:
S >= sqrt(I2t) / k = 395 / 143 = 2.76 mm^2 CSA minimum

If we had Ze = 0.7 ohms (double the fault impedance) then our current is half at 312A but then our disconnect time is 7s hence I2t = 312^2 * 7 = 681k (A2s units) which is about 4.4 times larger and now our minimum earth conductor is given by:
S >= sqrt(681E3) / 143 = 5.77 mm^2 CSA

For a fuse (at least our BS88 ones) they limit the I2t let-through and so worst-case is at lowest fault currents when disconnection takes a long time. Of course once you get in to the ten seconds or more the adiabatic assumption no longer holds so eventually you end up with a steady-state current carrying requirement. But for a fuse the least fault energy is at max PFC.

For a breaker it is more complicated, and you have a massive difference between faults that hit the "instantaneous" trip and those that don't. Also MCB and MCCB are not fault-limiting to the same degree as fuses, so as PFC increases from that trip point you see a moderate increase in let-through energy.
[automerge]1590337182[/automerge]
Just to add - that is why you do the lowest PFC (minimum supply voltage) for the adiabatic check as it gives the longest disconnect time and hence usually the biggest let-through energy.
 
Last edited:
I wish I could give this reply 100 likes. Love it!

I think I might use this to compare our tables, especially with motor OCPD sizing rules.
 
But how does this work out with sputtering or arcing faults?
Poorly I suspect.

It is always a bit of a gamble under-sizing the CPC on the assumption that any fault is a clean hard one that trips the OCPD as quickly as you assumed. For high current circuits an arc is likely, but that should mean you deduct tens or even a hundred volts from the supply to correct for the imperfect fault.

In the UK we don't do so much on arc-fault calculations, which is a big thing in the USA. Not sure if it is down to our systems generally having fast-disconnect or your systems having much higher PFC creating the greater risks, or just we don't see enough thermal injuries compared to shock to push it up the priority list.

But returning to CPC calculations, usually I would be conservative and go for the higher size (towards the phase/2 "rule-of-thumb") to help cope with such events. Also we do see RCD being used for fire control, but not as common generally (more of a thing for agricultural buildings, etc).

For completeness here are some example let-through curves for Hager B-curve MCB. As a point of reference the high fault limit for a 63A BS88 fuse is around 18k A2s (so at 10kA fault less than a 6A MCB) but from above you see the fault energy in the region from "instantaneous" trip to a few kA is much better in the MCB case:
Ohm value of transformer Hager-B-curve-MCB - EletriciansForums.net
 
FWIW, UL found that if a circuit's minimum bolted fault current was 125% greater than the breaker's magnetic pickup threshold the circuit was protected from parallel arc faults.

The UK's wiring regs already provide parallel arc fault protection in that values are adjusted by 80%:

View: https://youtu.be/5rHXBD9UYg4?t=365

Remember that wiring under full load wire doesn't actually reach 70*C.

On the other hand we don't assure disconnection times by any measure- so it was much easier for manufacturers to sell AFCIs to the US. It was EU breakers that actually got the ball rolling:


15 and 20 amp breakers in general got a magnetic pickup of around 150 amps:

 
Last edited:
FWIW, UL found that if a circuit's minimum bolted fault current was 125% greater than the breaker's magnetic pickup threshold the circuit was protected from parallel arc faults.

The UK's wiring regs already provide parallel arc fault protection in that values are adjusted by 80%
Yes, that would be reasonable for a clean fault.

Remember that wiring under full load wire doesn't actually reach 70*C.

On the other hand we don't assure disconnection times by any measure- so it was much easier for manufacturers to sell AFCIs to the US. It was EU breakers that actually got the ball rolling:

15 and 20 amp breakers in general got a magnetic pickup of around 150 amps:
Some systems run closer to max temperature than others, but unlikely in most cases for sure. I remember seeing that paper before, really quite a surprise. Do USA breakers now have something closer to the EU style of 3-5 In for the magnetic trip?

A lot of the Zs requirements are being eroded by widespread RCD use, but I am always a bit paranoid about electronics failing so prefer to know it would clear OK on the MCB trip alone!
 
Yes, that would be reasonable for a clean fault.

UL says it will take care of a parallel arc fault. Our AFCIs stop looking for parellel arcs below 75 amps as this was the lowest short circuit value determined (theorized really) to be found in a US home's in-wall wiring- after research was conducted on dwelling properties by UL.


Some systems run closer to max temperature than others, but unlikely in most cases for sure. I remember seeing that paper before, really quite a surprise.

A lot of people are surprised or straight up don't believe me when I compare the NEC with VDE, BS7671, ect.

The thing is the NFPA is purely a reactionary, litigation responsive document. Change typically does not come about unless something can be proven well beyond a doubt as presenting a hazard- usually after one or more documented tragedies.

For example, the NEC specifically added verbiage forbidding the earth (TT) as being used as an effective ground fault current path and re-word the definitions of grounding, bonding and "effective ground fault current path" after numerous incidents where ground rods alone failed to clear a fault.

Still then manufacturer reps sitting on code making panels have the final word, often voting on specific products instead of rules rooted in electrical theory.

Do USA breakers now have something closer to the EU style of 3-5 In for the magnetic trip?

Its around 6-10x for today's for single pole 15 and 20 amp breakers. Originally the idea was to have all North American 15 and 20 amp single pole breakers trip at 75 amps, but discovered such a threshold would result in nuisance tripping from appliance inrush. This resulted in electronic AFCIs being developed, tested and then mandated by NFPA 70.

The thing is NFPA-70 could have just mandated loop impedance calcs which would have done the job just as well with today's 6-10x breakers which aren't causing any problems, even with highly inductive loads like refrigerators, washing machines, and microwave ovens (which typically lack inrush suppression in North America).


A lot of the Zs requirements are being eroded by widespread RCD use, but I am always a bit paranoid about electronics failing so prefer to know it would clear OK on the MCB trip alone!


Never trust electronics. Your thinking is correct. An RCD is nothing more than a backup CPC- and a rather wonky one.

The 2020 NEC just mandated surge protection devices on all dwelling unit services and has been doing so for several code cycles in regards to emergency circuits. Under 2020 practically every single dwelling unit circuit requires either an AFCI, GFCI, or dual function GFCI/AFCI breaker.

GFCIs were mandated in the US due to the large number of two prong metal framed tools which were becoming live while in use. GFCI latter expanded to other scenarios where EGCs were being compromised like cord and plug connected pool pump motors.

EU has a variety schuko plugs which can mate with each other but not always with an earth connection. Thus RCDs were a good idea in the EU.

The UK was able to hold off into the 90s due to the fact all none double insulted tools and appliances were equipped with an EGC since WWII, and a missing earth pin would not open the socket shutters and not likely be broken in the first place due the robust design of UK plugs.
 
The thing is the NFPA is purely a reactionary, litigation responsive document. Change typically does not come about unless something can be proven well beyond a doubt as presenting a hazard- usually after one or more documented tragedies.
Sadly that is not a surprise to folk on this side of the pond.

For example, the NEC specifically added verbiage forbidding the earth (TT) as being used as an effective ground fault current path and re-word the definitions of grounding, bonding and "effective ground fault current path" after numerous incidents where ground rods alone failed to clear a fault.
It has long been the case here that a TT setup would (to all practical purposes) need an RCD-style of device as the rod impedance is very unlikely to be low enough for operation of the OCPD.

Yes, up until probably the 70s they were the VOELCB style that were not particularly reliable in operation (not the units themselves, but the means of detection).

Its around 6-10x for today's for single pole 15 and 20 amp breakers. Originally the idea was to have all North American 15 and 20 amp single pole breakers trip at 75 amps, but discovered such a threshold would result in nuisance tripping from appliance inrush. This resulted in electronic AFCIs being developed, tested and then mandated by NFPA 70.

The thing is NFPA-70 could have just mandated loop impedance calcs which would have done the job just as well with today's 6-10x breakers which aren't causing any problems, even with highly inductive loads like refrigerators, washing machines, and microwave ovens (which typically lack inrush suppression in North America).
So they are similar to our C-curve MCBs (5-10 * In) then.

I guess in the UK we have the advantage of the ring & fused plug system, so our 32A B-breaker has a similar instantaneous trip point of 100-150A, but individual end appliances are often on a smaller fuse such as 13A for washing machine, 5A for may electronics devices like TVs, etc, and hence a lot less fault energy if they short out.

Never trust electronics. Your thinking is correct. An RCD is nothing more than a backup CPC- and a rather wonky one.
Here it is often referred to as 'additional protection' which is a good way of thinking about it - it is there for shock protection more than fault clearing.

But the regulations and a few design tools are using the RCD trip Zs as the design test, and not the OCPD requirement. Now in something like the TT incomer RCD case you have to accept that, but if it were down to me I would adjust the wording of the regs a bit to make it clear you should always design for the OCPD's Zs and only in justifiable cases where it is not feasible to meet that to then rely on RCD action in its place.
 
Sadly that is not a surprise to folk on this side of the pond.

I'm just glad there is awareness of it.


It has long been the case here that a TT setup would (to all practical purposes) need an RCD-style of device as the rod impedance is very unlikely to be low enough for operation of the OCPD.

Yes, up until probably the 70s they were the VOELCB style that were not particularly reliable in operation (not the units themselves, but the means of detection).

Yup- though the high Z of earth rods was not readily known in the 70s.


So they are similar to our C-curve MCBs (5-10 * In) then.

Yes, similar. Two and 3 pole breakers are closer to a D curve in order to allow inrush on large multi-motor equipment.

If that is not enough we can also use Table 430.52 and increase the size of the breaker up to 250%, 175% for time delay fuses and 300% for none time delay fuses.


View: https://Upload the image directly to the thread.com/9Gh5YsY


As an example 2.08mm2 copper line and earth can be protected with a 40 amp breaker. :eek:

I guess in the UK we have the advantage of the ring & fused plug system, so our 32A B-breaker has a similar instantaneous trip point of 100-150A, but individual end appliances are often on a smaller fuse such as 13A for washing machine, 5A for may electronics devices like TVs, etc, and hence a lot less fault energy if they short out.

Trust me, UL had fun studying your system when developing AFCIs. Damaged cords rather quickly blow plug fuses.

Here it is often referred to as 'additional protection' which is a good way of thinking about it - it is there for shock protection more than fault clearing.

But the regulations and a few design tools are using the RCD trip Zs as the design test, and not the OCPD requirement. Now in something like the TT incomer RCD case you have to accept that, but if it were down to me I would adjust the wording of the regs a bit to make it clear you should always design for the OCPD's Zs and only in justifiable cases where it is not feasible to meet that to then rely on RCD action in its place.

I'm with you on this. Plus you still have L-N faults.
 
Yup- though the high Z of earth rods was not readily known in the 70s.
Oh I think the impedance of earth rods was known about in the 19th century!

Actually applying that to regulations and safe design, well...

As an example 2.08mm2 copper line and earth can be protected with a 40 amp breaker. :eek:
That is one of the marginal cases here, it would be acceptable only if the fault-clearing time is low enough to limit the energy. Which is back to Zs once more.

Generally we have two cases of related protection:
  • Short circuit protection
  • Overload protection
Short circuit protection is always required, with the exception of situations where the source is fundamentally current limited (e.g. some bell transformers, etc) or it is a short run and well protected (e.g. bus bar tap-off cables where a fault in that area is seen as unlikely)

Overload protection is not always required at the source end of a cable, as the load might provide that (if a fixed demand appliance, or cable terminates in a switch/fuse or a DB with a limited total current).

I'm with you on this. Plus you still have L-N faults.
Generally we treat that as above, and we look to Zs disconnection as worst-case.

That is not always true, as you might find parallel earth paths making PFC higher than PSSC, but if you have designed you L+E wires to meet disconnection times then L-N faults should be a slam-dunk (to borrow one of your phrases).

But going down the RCD path makes that less certain, though we do still have the volt-drop limit acting as another sanity check to cause fast disconnection under short conditions (e.g. If you are designing a 20A circuit with 5% max VD then your PSSC should be at least 20A / 0.05 = 400A at nominal voltage which is enough for even a D-curve breaker's instantaneous trip). But it is not as clearly defined as for the shock-protection case of L-E disconnection times, and is worded more in terms of conductor protection (and even for the L-E times, there is a note that faster than 0.4s/5s may be required for thermal reasons).

So in a sense we have all of the right bits in place, but personally I would like to see the OCPD requirement on max Zs pushed more in the RCD cases.
 
Last edited:
Oh I think the impedance of earth rods was known about in the 19th century!

Actually applying that to regulations and safe design, well...

I'm not sure why it took 60 years for them to realize it. Its possible the constant use of the word "ground" "ground wire" "ground fault" the code gave rise to that myth... but I have no idea to be honest.

That is one of the marginal cases here, it would be acceptable only if the fault-clearing time is low enough to limit the energy. Which is back to Zs once more.

Generally we have two cases of related protection:
  • Short circuit protection
  • Overload protection
Short circuit protection is always required, with the exception of situations where the source is fundamentally current limited (e.g. some bell transformers, etc) or it is a short run and well protected (e.g. bus bar tap-off cables where a fault in that area is seen as unlikely)

Overload protection is not always required at the source end of a cable, as the load might provide that (if a fixed demand appliance, or cable terminates in a switch/fuse or a DB with a limited total current).

NEC is much the same way. It distinguishes between overload and short circuit protection. Motors and AC equipment are considered as having overload protection at the load end. Welders are allowed to be on smaller conductors relative to the OCPD rating (duty cycle exception). Circuits with fixed equipment can go the "next standard size up" in regards to OCPD selection. Same for calculated loads like sub-panels. 55 amp wire can go on a 60 amp breaker, ect.

In Canada (which uses the CEC, a close relative of NFPA-70) fixed electric heat circuits can take an OCPD 125% of the final circuit conductor rating.

The difference being no Z requirements.





Generally we treat that as above, and we look to Zs disconnection as worst-case.

That is not always true, as you might find parallel earth paths making PFC higher than PSSC, but if you have designed you L+E wires to meet disconnection times then L-N faults should be a slam-dunk (to borrow one of your phrases).

Borrow what you need :) Its the smallest favor I can re-turn.

But going down the RCD path makes that less certain, though we do still have the volt-drop limit acting as another sanity check to cause fast disconnection under short conditions (e.g. If you are designing a 20A circuit with 5% max VD then your PSSC should be at least 20A / 0.05 = 400A at nominal voltage which is enough for even a D-curve breaker's instantaneous trip). But it is not as clearly defined as for the shock-protection case of L-E disconnection times, and is worded more in terms of conductor protection (and even for the L-E times, there is a note that faster than 0.4s/5s may be required for thermal reasons).

So in a sense we have all of the right bits in place, but personally I would like to see the OCPD requirement on max Zs pushed more in the RCD cases.

You're way ahead of use. The thing is the NFPA knows that, gradually mandating GFCIs on all circuits... something we do not all agree with.
 

Reply to Ohm value of transformer in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock